SINGLE-PHASE 



291 



the conductors do not cross one another, etc. However, it is well 

 known that a drum winding, for the same number of poles, has 

 the same electrical characteristics as the gramme-ring winding. 

 The preceding analysis applies equally well to a drum-wound 

 armature. Also, the foregoing principles apply to motors of 

 more than two poles. Figure 267 shows the brush positions for 

 a four-pole motor. 



Instead of displacing the brushes from the geometrical neutral 

 so that a potential difference exists between them, which results 

 ma current, giving rise to torque, the same effect may be obtained 

 by using two field windings displaced at right angles to each 

 other, as shown in Fig. 268. A compensating or transformer 



Compensating 



or 



Transformer 

 Field 



i-polc repulsion 

 motor. 



Fio. 268. Two-pole repulsion motor with 

 compensating or transformer field. 



field, acting along the brush axis, induces emt's.. which in turn 

 cause curn-nts, shown in Fig. 265, and these currents react with 

 the flux of the main field winding to produce torque. This type 

 of motor should not be confused with the four-pole type of 1 ILL. 



IVaetieally all repulsion motors are made with non-<alient 

 han with the salient pole- -howii in the diairramma- 

 tic illustrations just given. The windings are usually of the 

 distributed type, such as are used for induction motors. The 

 fact that the reluctance to the main-field flux and to the trans- 

 former-field flux mu>1 be kept a.S low as possible inak Table 

 to use non-salient polo and to make the air-gap as slio 

 sible. Otherwise, th. rents for the.-e field.- will be 

 high, lou-erinu the power-factor. 



